An optical wavelength filter is a device that reflects or transmits light of a desired wavelength or within a certain wavelength range. For example, an interference bandpass filter selectively transmits light within a selected wavelength transmission bandwidth while absorbing light of wavelengths outside the transmission bandwidth. Such optical filtering with respect to wavelength provides a means of controlling the energy and spectral composition of light and is widely used in a variety of optical signal processing, detection, and display applications.
Excitation of surface plasmon waves at a metal-dielectric interface has been demonstrated as an efficient way of implementing a spectral filtering mechanism in response to an electrical control signal. See, for example, Wang and Simon, “Electronic Reflection with Surface Plasmon,” Opt. Quantum Electron.25, S925 (1993) and Wang, “Voltage-Induced Color-Selective Absorption with Surface Plasmon”, Appl. Phys. Lett. 67, pp. 2759-2761 (1995). Surface plasmon are oscillations of free electrons caused by resonant absorption of a p-polarized incident optical wave at a metal-dielectric interface when the wavelength and incident angle of the optical wave satisfy a plasmon resonance condition. More specifically, the plasmon resonance condition requires that the component of the optical wave vector along the metal-dielectric interface matches the plasmon wave vector, Kp:             K      p        =                            2          ⁢          π                λ            ⁢                                                  ɛ              1                        ⁢                          ɛ              2                                                          ɛ              1                        +                          ɛ              2                                            ,where, λ is the wavelength of the optical wave, ε1 and ε2 are the dielectric permittivity constants for the metal and the dielectric material, respectively.
At surface plasmon resonance, the energy of the incident optical wave is strongly absorbed and converted into the energy of oscillating free electrons in the metal. Therefore, the reflected optical wave is strongly attenuated or even vanishes. When the incident angle of the optical wave is fixed at a constant, the optical wavelength λ satisfying the plasmon resonance condition may be changed by varying the dielectric permittivity constant ε2 of the dielectric material. If the input optical wave is white light, the color of the reflected optical wave will change with ε2. This phenomena effects a surface plasmon tunable filter in reflection mode.
Therefore, an electronically tunable filter can be formed by using an electro-optic material as the dielectric material. The voltage applied on the electro-optic material changes its index of refraction and thereby changes the wavelength for the surface plasmon resonance.
Wang and Simon disclose color display devices based on a surface plasmon filter using a liquid crystal electro-optic material. U.S. Pat. Nos. 5,451,980 and 5,570,139, which are incorporated herein by reference. The index of the refraction of the liquid crystal is changed by applying a voltage to alter the spectral composition of the reflected light.